Another recent innovation in the design of stoves, especially those fashioned for cooking in the home, has been the development of so-called smoothtop cooking ranges. For the most part, a sheet of glass-ceramic material has comprised the cooking surface of such ranges with electrical heating elements being placed into contact with, or in close proximity to, the underside of the sheet. Cooking utensils are placed upon the glass-ceramic sheet over the heating unit positions.
Glass-ceramics are produced via the crystallization of glass articles. The method of producing such articles comprehends three general steps. First, a glass-forming batch, to which a nucleating agent is frequently added, is melted. Second, the melt is simultaneously cooled to at least below the transformation range thereof and shaped into a glass body of a desired configuration. Third, the glass body is subjected to a predetermined heat treatment to cause the glass to crystallize in situ. Commonly, the heat treatment step is divided into two elements. That is, the glass body is first heated to a temperature within or somewhat above the transformation range to initiate nucleation in the glass. Thereafter, the glass is heated to a higher temperature, frequently a temperature in excess of the softening point of the glass, to effect growth of crystals upon the previously-developed nuclei.
Since the crystallization mechanism involves the essentially simultaneous growth of crystals upon innumerable nuclei dispersed throughout the parent glass body, the microstructure of a glass-ceramic article normally consists of relatively uniformly-sized, fine-grained crystals homogeneously distributed, but randomly oriented, within a residual glassy matrix. Conventionally, the crystal phase will comprise the predominant proportion of the glass-ceramic, i.e., the crystal phase will constitute greater than 50% by volume thereof. Such high crystallinity customarily results in glass-ceramic articles displaying chemical and physical properties quite dissimilar from those of the precursor glass body and more closely akin to those of the crystal phase. Moreover, because the crystal phase commonly forms the majority of a glass-ceramic, the residual glassy matrix will necessarily be small in quantity and quite different in composition from the parent glass, inasmuch as the components making up the crystal phase will have been removed from the glass during the crystallization process. Also, the presence of a residual glassy matrix renders a glass-ceramic free from voids and non-porous.
U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, contains a detailed description of the mechanism of crystal growth and a study of the several practical considerations inherent in the manufacture of glass-ceramics. Reference is hereby made to that patent for a basic understanding that the crystal phases developed in glass-ceramic articles and the amount of such crystallinity are functions of parent glass composition and the heat treatment parameters to which the parent glass is subjected.
Customarily, the glass-ceramic sheet comprising the smoothtop cooking surface has been opaque and, most generally, has been white in color. The opacity of the material was considered desirable since it concealed the heating units from view. The top or cooking surface of the glass-ceramic sheet has customarily been decorated, at least in those areas above the heating elements, so as to indicate their positions.
This use of an opaque sheet, however, has given rise to a safety hazard in that the surface provides no visible evidence that the heating elements are in operation. Accordingly, persons using the stove may be harmed by inadvertently contacting the smoothtop surface in an area where the operating heating elements are located. Of course, the ranges conventionally have warning lights on the dial or other means for activating the heating units, but accidental burnings are still frequently reported.
Consequently, considerable research has been conducted to develop a glass-ceramic sheet especially suitable for use as smoothtop cooking surfaces which would display an overall pleasant aesthetic appearance and which would conceal the underlying heating units from view when not in operation, but which would provide a visible indication when the heating elements are in use. Laboratory experimentation demonstrated that glass-ceramic sheets could be devised which were sufficiently translucent to mask the heating units when not in operation but would transmit an outline of the elements when functioning. Transparent glass-ceramics were also produced which were sufficiently highly saturated to accomplish the same purpose. Unfortunately, each of those products was subject to a marketing drawback.
First, field testing and consumer surveys indicated that a translucent sheet was not looked upon with much favor by a majority of the persons interviewed. The inherent hazy appearance of the translucent sheet, particularly in white-colored bodies, was considered objectionable.
Second, the transparent materials frequently exhibited mechanical strengths which were unacceptable or, at best, marginal for stove top applications unless the sheets were subjected to a separate and additional strengthening treatment, e.g., chemical strengthening through an ion exchange reaction in a bath of molten salts.
As a result of the above field tests and consumer surveys, a transparent material which would have a warm brown coloration sufficiently dark to conceal the heating element when not in use, but wherein the glow of the unit can be seen in operation, was adjudged to be the most acceptable. For use as stove top surfaces, laboratory and field experience has indicated that the material should exhibit a modulus of rupture of at least 8000 psi and, most preferably, in excess of 14,000 psi.